1. Field of the Invention
The present invention relates to a power amplifier and an operating method of the power amplifier, and more particularly relates to a technique that is effective to enable power amplification in a wide range from a low power state to a high power state and to increase power added efficiency when power of RF input signals having transmit frequencies in a plurality of frequency bands is amplified.
2. Description of the Related Art
A mobile communication device terminal that is operated by a battery, such as a cellular phone, must increase the power efficiency of a power amplifier that transmits an RF transmit signal to a base station. Additionally, the power consumption of the power amplifier has to be reduced to provide as long a talk time as possible through a single charge of the battery.
Japanese Unexamined Patent Application Publication No. 2008-035487 describes that a first amplifying element with a small element size and a second amplifying element with a large element size are connected in parallel, power amplification is executed by the first amplifying element in a low power state, and power amplification is executed by the second amplifying element in a high power state. The first amplifying element with the small element size exhibits high power added efficiency (PAE) in the low power state, and the second amplifying element with the large element size exhibits high power added efficiency (PAE) in the high power state. Hence, the power added efficiency of the power amplifier can be increased for transmission of power in a wide range from the low power state to the high power state. The U.S. patent publication corresponding to Japanese Unexamined Patent Application Publication No. 2008-035487 is U.S. Patent Application Publication No. 2007/0298736 A1.
U.S. Pat. No. 7,157,966 describes that a first output stage with the element size of an output transistor being optimized for high power and a second output stage with the element size of an output transistor being optimized for low power are connected in parallel, and a bias control circuit selects the first output stage in the high power and selects the second output stage in the low power. The first output stage and the second output stage are connected to a single output impedance matching circuit. The single output impedance matching circuit includes a plurality of capacitances and a plurality of inductors.
The inventors of the present invention participated in development to provide a power amplifier that could be mounted on a cellular phone of the third generation (3G) or the fourth generation (4G) and could perform transmission in a plurality of frequency bands. Further, this power amplifier had to have high power added efficiency (PAE) to provide a long talk time.
FIG. 23 illustrates a configuration of a power amplifier PA studied by the inventors of the present invention prior to development of the present invention.
The power amplifier PA shown in FIG. 23 and studied by the inventors of the present invention prior to development of the present invention includes a first amplifier 1 with a large element size, a second amplifier 2 with a small element size, a first output matching circuit 3, and a second output matching circuit 4.
The first amplifier 1 functions as a main amplifier including a transistor with a large element size to exhibit high power added efficiency (PAE) in a high power state, and the second amplifier 2 functions as a sub amplifier including a transistor with a small element size to exhibit high power added efficiency (PAE) in a low power state.
If the first amplifier 1 is operated, a first amplifier enable signal is supplied to a first control terminal 201. If the second amplifier 2 is operated, a second amplifier enable signal is supplied to a second control terminal 202.
An input terminal of the first amplifier 1 and an input terminal of the second amplifier 2 are commonly connected to an RF signal input terminal 101 of the power amplifier PA. AN RF input signal that is supplied to the RF signal input terminal 101 is amplified by the first amplifier 1 or the second amplifier 2.
A ground terminal of the first amplifier 1 and a ground terminal of the second amplifier 2 are commonly connected to a ground terminal of the power amplifier PA. A ground electrode of the power amplifier PA is electrically connected to ground wiring of a mother board of a cellular phone with a very small wiring resistance. Hence, the first amplifier 1 and the second amplifier 2 execute a markedly stable RF amplification operation. Further, the ground terminal of the power amplifier PA is mechanically connected to the ground wiring of the mother board of the cellular phone with a very small thermal resistance. Hence, Joule heat that is generated from the first amplifier 1 and the second amplifier 2 can be effectively radiated from the mother board of the cellular phone.
An output terminal of the first amplifier 1 is connected to an input terminal of the first output matching circuit 3, and an output terminal of the second amplifier 2 is connected to an input terminal of the second output matching circuit 4. An output terminal of the second output matching circuit 4 is connected to the input terminal of the first output matching circuit 3, and an output terminal of the first output matching circuit 3 is connected to an output terminal 102 of the power amplifier PA.
In the first amplifier 1, an input electrode and a ground electrode of a transistor Q1 with a large element size are respectively connected to the input terminal and the ground terminal of the first amplifier 1. An output electrode of the transistor Q1 with the large element size is connected to a power terminal 205 of the power amplifier PA through a first load. The output electrode of the transistor Q1 with the large element size is connected to the output terminal of the first amplifier 1.
In the second amplifier 2, an input electrode and a ground electrode of a transistor Q2 with a small element size are respectively connected to the input terminal and the ground terminal of the second amplifier 2. An output electrode of the transistor Q2 with the small element size is connected to a power terminal 205 of the power amplifier PA through a second load. The output electrode of the transistor Q2 with the small element size is connected to the output terminal of the second amplifier 2.
The first amplifier 1 has a relatively small output impedance because of the transistor Q1 having the large element size of the first amplifier 1. In contrast, the second amplifier 2 has a relatively large output impedance because of the transistor Q2 having the small element size of the second amplifier 2. For example, the output impedance of the first amplifier 1 is several ohms, and the output impedance of the second amplifier 2 is several tens of ohms.
The first output matching circuit 3, the input terminal of which is connected to the output terminal of the first amplifier 1, executes matching between the output impedance of several ohms of the first amplifier 1 and an impedance of a transmit antenna of 50Ω, which is connected to the output terminal 102 of the first output matching circuit 3. That is, an input impedance of the first output matching circuit 3 is set at several ohms, and hence the output impedance of the first amplifier 1 and the input impedance of the first output matching circuit 3 are matched. Consequently, reflection of the RF signal can be sufficiently reduced between the output of the first amplifier 1 and the input of the first output matching circuit 3. Further, an output impedance of the first output matching circuit 3 is set at 50Ω, and hence the output impedance of the first output matching circuit 3 and the input impedance of the transmit antenna of 50Ω are matched. Consequently, reflection of the RF signal can be sufficiently reduced between the output of the first output matching circuit 3 and the input of the transmit antenna. The first output matching circuit 3 can be provided by a plurality of inductors and a plurality of capacitances.
The second output matching circuit 4 executes matching between the output impedance of several tens of ohms of the second amplifier 2 and the input impedance of several ohms of the first output matching circuit 3. That is, an input impedance of the second output matching circuit 4 is set at several tens of ohms, and hence the output impedance of the second amplifier 2 and the input impedance of the second output matching circuit 4 are matched. Consequently, reflection of the RF signal can be sufficiently reduced between the output of the second amplifier 2 and the input of the second output matching circuit 4. Further, an output impedance of the second output matching circuit 4 is set at several ohms, and hence the output impedance of the second output matching circuit 4 and the input impedance of the first output matching circuit 3 are matched. Consequently, reflection of the RF signal can be sufficiently reduced between the output of the second output matching circuit 4 and the input of the first output matching circuit 3.
The second output matching circuit 4 of the power amplifier PA shown in FIG. 23 and studied by the inventors of the present invention prior to development of the present invention is provided by an inductor L1, a capacitance C1, and a first switch (SW1) 60. One end of the inductor L1 is connected to the output terminal of the first amplifier 1, the input terminal of the first output matching circuit 3, and the output terminal of the second output matching circuit 4. The other end of the inductor L1 is connected to the output terminal of the second amplifier 2 and the input terminal of the second output matching circuit 4. One end of the capacitance C1 is connected to the output terminal of the second amplifier 2 and the input terminal of the second output matching circuit 4. The other end of the capacitance C1 is connected to a ground potential GND.
If the second amplifier 2 is brought into a non-active state by the second amplifier enable signal that is supplied to the second control terminal 202, the first switch (SW1) 60 reduces an effect of the second output matching circuit 4 to the output terminal of the first amplifier 1. In the example shown in FIG. 23, the first switch (SW1) 60 is connected between the other end of the inductor L1 and the one end of the capacitance C1.
FIG. 24 illustrates a Smith chart explaining an operation of the second output matching circuit 4 of the power amplifier PA studied by the inventors of the present invention prior to the present invention shown in FIG. 23.
In FIG. 24, for easier understanding of the operation, it is assumed that an output impedance Zout_SA of the second amplifier 2 is a larger value than the actual several tens of ohms.
In FIG. 24, a relatively small output impedance Zout_MA of the first amplifier 1 and a relatively large output impedance Zout_SA of the second amplifier 2 are indicated on a line that connects a point at which a resistance value is zero (0) and a point at which the resistance value is infinity (∞). The relatively small output impedance Zout_MA of the first amplifier 1 is located at a slightly right side with respect to the point at which the resistance value is the zero (0). The relatively large output impedance Zout_SA of the second amplifier 2 is indicated at the right side of a point at which the resistance value is 50Ω; however, the output impedance Zout_SA is actually located between a point at which the resistance value is 25Ω and the point at which the resistance value is 50Ω.
Further, FIG. 24 indicates an input impedance Zin_MN of the first output matching circuit 3. The input impedance Zin_MN is matched to an impedance that is substantially equivalent to the relatively small output impedance Zout_MA of the first amplifier 1.
Hence, if the first amplifier 1 is controlled to be in a non-active state by the first amplifier enable signal that is supplied to the first control terminal 201, and if the second amplifier 2 is controlled to be in an active state by the second amplifier enable signal that is supplied to the second control terminal 202, the second output matching circuit 4 has to execute an impedance matching operation between the output impedance Zout_SA of the second amplifier 2 and the input impedance Zin_MN of the first output matching circuit 3 as follows.
That is, with the inductor L1 of the second output matching circuit 4, an impedance ZL at the other end of the inductor L1 departs from the input impedance Zin_MN of the first output matching circuit 3 and moves clockwise on an arc of a constant resistance circle. The moving amount at this time becomes ωL1 corresponding to an impedance jωL1 of the inductor L1. Herein, ω is an angular frequency.
Further, with the capacitance C1 of the second output matching circuit 4, the impedance at one end of the capacitance C1 departs from the impedance ZL at the other end of the inductor L1 and moves clockwise on an arc of a constant-conductance circle. The moving amount at this time becomes ωC1 corresponding to an admittance jωC1 of the capacitance C1.
Hence, the destination obtained by the sum total of the moving amount ωL1 of the inductor L1 and the moving amount ωC1 of the capacitance C1 has to be matched to the output impedance Zout_SA of the second amplifier 2. With this matching, the output impedance Zout_SA of the second amplifier 2 and the input impedance Zin_MN of the first output matching circuit 3 can be matched by the second output matching circuit 4 with almost no loss.
The power amplifier PA shown in FIG. 23 and studied by the inventors of the present invention prior to development of the present invention was designed to amplify an RF input signal RFIN having a transmit frequency in a single frequency band. Hence, the second output matching circuit 4 shown in FIG. 24 was designed to match the output impedance Zout_SA of the second amplifier 2 and the input impedance Zin_MN of the first output matching circuit 3 with the transmit frequency in the single frequency band.
However, since the power amplifier PA studied by the inventors of the present invention prior to development of the present invention shown in FIG. 23 was to be mounted on cellular phones of the third generation (3G) and the fourth generation (4G), RF input signals RFIN having transmit frequencies in a plurality of frequency bands had to be amplified by the power amplifier PA shown in FIG. 23.
A transmit frequency in a single frequency band was initially a high transmit frequency fHB among transmit frequencies in a plurality of frequency bands.
Hence, if the RF input signal RFIN having the high-band transmit frequency fHB is amplified by the power amplifier PA shown in FIG. 23, as shown in FIG. 24, by using the second output matching circuit 4 shown in FIG. 23, the destination obtained by the sum total of the moving amount ωL1 of the inductor L1 and the moving amount ωC1 of the capacitance C1 could be matched to the output impedance Zout_SA of the second amplifier 2.
In contrast, if the RF input signal RFIN having the low-band transmit frequency fLB is amplified by the power amplifier PA shown in FIG. 23, as shown in FIG. 24, even by using the second output matching circuit 4 shown in FIG. 23, the destination obtained by the sum total of the moving amount ωL1 of the inductor L1 and the moving amount ωC1 of the capacitance C1 could not be matched to the output impedance Zout_SA of the second amplifier 2. This is because the moving amount ωL1 of the inductor L1 and the moving amount ωC1 of the capacitance C1 are decreased with the low-band transmit frequency fLB. As the result of the study by the inventors of the present invention prior to development of the present invention, a problem is recognized in which, with the mismatching between impedances, if the RF input signal applied to the input terminal 101 and having the low-band transmit frequency fLB is amplified by the power amplifier PA shown in FIG. 23, the power added efficiency (PAE) is decreased.